This invention relates to a composition and process for forming a storage-stable, one package, blocked polyurethane which is highly compatible with thermoplast plastisols such as polyvinylchloride (PVC) and polymethylmethacrylate (PMMA) plastisols.
Thermoplast plastisols are commonly used in the automotive industry as sealants and underbody coatings. A preferred use is as a secondary coating applied over a primary coating formed by electrodeposition (E-coat). The secondary coating provides additional corrosion protection and coverage over areas missed by the electrodeposition process.
In order to develop the full strength of the plastisols so that they can be useful, they generally require baking temperatures of 130.degree. C. or higher. Due to the increased use of automotive plastic parts which have lower service temperatures and also for energy conservation purposes, the recent trend has been to drop the oven baking temperatures in automotive plants to 120.degree. C. or lower. At these lower baking temperatures, plastisols that are storage stable cannot develop their full strength due to poor fluxing (i.e., plasticization), and hence they are poorly adherent to E-coats and generally have poor abrasion resistance. Plastisols that can develop full strength at low bake temperatures, however, do not provide any adhesion and generally are not storage stable.
In order to improve processing characteristics, it is known in the art to blend thermoplast plastisols with polyurethanes. The polyurethanes used, however, are not highly compatible with the plastisols, are generally not storage-stable, and do not provide improved abrasion resistance and adhesion when the blends are baked at lower temperatures such as 120.degree. C.
Examples of blocked isocyanate compositions can be found in various patents. These include U.S. Pat. No. 3,583,943, which teaches a curable coating composition comprising (a) a ketoxime-blocked, isocyanate terminated reaction product of a polyhydroxyl compound with an isocyanate, the reaction product having a molecular weight of from about 8,000 to about 15,000 and (b) a tertiary amino alcohol having at least two hydroxyl groups.
U.S. Pat. No. 3,694,389 teaches a thermosetting coating composition comprising a ketoxime-blocked isocyanate and a solution copolymer of a mixture of ethylenically unsaturated and active hydrogen-containing compounds. U.S. Pat. No. 3,660,355 teaches novel thermosetting resin powders which can be molded to form urethane cross-linked products which are prepared by reacting hydroxy-functional, acrylic polymer particles with a mono-blocked diisocyanate. U.S. Pat. No. 3,676,402 teaches octaalkylstannoxanes as regenerative agents of an isocyanate group and a blocked isocyanate compound at temperatures lower than previously possible.
U.S. Pat. No. 3,857,818 teaches a powdered coating composition comprising a free flowing mixture of a solid, ketoxime-blocked isocyanate prepolymer and one or more solid, hydroxy functional resins such as a polyester resin. U.S. Pat. No. 3,984,365 teaches an aqueous solution of bisulfate-blocked polyisocyanate prepolymers having storage stability up to about 45.degree. C. by incorporation of about 0.5 to 20% by weight of the prepolymer of an aromatic or alkyl aromatic, sulfonic acid. U.S. Pat. No. 4,046,744 teaches a low temperature, one component, thermosetting coating composition comprising a ketoxime-blocked polyisocyanate and an oxazolidine.
U.S. Pat. No. 4,403,086 teaches high solid, solvent-based thermosetting resin coating compositions comprising chain-extendable, cross-linkable, low molecular weight polyol, deblocked diisocyanate and cross-linking agent reactive with a polyol, but substantially unreactive with the isocyanate functionality. U.S. Pat. No. 4,409,340 teaches a heat curable, coating composition comprising a prepolymer with ketoxime-blocked NCO groups which are linked to the prepolymer via cycloaliphatic and/or aliphatic groups, a diamine cross-linker and an organic solvent. U.S. Pat. No. 4,373,081 teaches thermally cross-linkable, coating compositions which are liquid and storage stable at room temperature and which contain combinations of dialkyl-malonate-blocked polyisocyanates and organic polyhydroxyl compounds as binders.
U.S. Pat. No. 4,696,991 relates to blocked isocyanurates which are curable at low temperatures to form coatings. The blocking agent can be an oxime.
Patents disclosing low temperature cure include U.S. Pat. No. 4,546,166 and German Offenlegungsschrift DE No. 32 28 670. However, most of the compositions cited do not provide sufficient shelf life. For instance, in U.S. Pat. No. 4,546,166, although it is disclosed that the polyurethane composition can be cured at 80.degree.-100.degree. C., the cure time is two to four hours and the composition is not storage stable because the polyisocyanates used are not blocked. In a case where storage stability is disclosed (for 2 months at room temperature) in German Offenlegungsschrift DE No. 32 28 670, the curing temperature is significantly higher, i.e., 140.degree. C., for 20 minutes.
Blocked isocyanates prepared from m-xylylene diisocyanate are known as taught by U.S. Pat. No. 4,824,925. This patent discloses blocked isocyanate curing agents prepared by reacting m-xylylene diisocyanate with a ketoxime. The curing agents are used in combination with active hydrogen-containing materials to form curable compositions such as coatings. This patent does not teach combining a blocked polyurethane prepolymer with a plastisol.
U.S. Pat. No. 4,623,686 relates to coating compositions which include polyvinyl plastisols, polyisocyanates with retarded activity, and one or more plasticizers. The polyisocyanates are used as bonding agents for the plastisols.
The use of an amine curing agent is taught by U.S. Pat. No. 4,624,996. This patent teaches a one package polyurethane composition in which a polyurethane prepolymer is blocked with an oxime. The prepolymer can be formed from one or more polyisocyanates. None of the polyisocyanates mentioned are hindered polyisocyanates and there is no disclosure that the use of hindered polyisocyanates provides any improved result. This patent also does not disclose the use of thermoplast plastisol compositions.
Prior art systems have been developed which improve the performance of PVC plastisols at lower curing temperatures. For example, U.S. Pat. No. 4,722,969 relates to blocked urethane compositions which, when blended with plastisols, allows curing of plastisol coatings at temperatures of 100.degree.-120.degree. C. for 10-30 minutes. In addition, the blends are storage stable for at least 3 days at 40.degree. C. or for 2 months at room temperature, i.e., at 25.degree. C. The polyurethanes of this patent are prepared from a branched isocyanate prepolymer blocked with a ketoxime blocker, a polyol cross-linking agent, a deblocking and curing catalyst and a desiccant to prevent foaming.
While plastisol blends prepared with the polyurethane compositions of U.S. Pat. No. 4,722,969 exhibit good and in some cases outstanding performance, there remains room for improvement in some respects. For example, the polyurethane compositions of this patent are not 100% compatible with PVC plastisols. Also, the use of an amine curing agent results in inadequate shelf life.
There thus remains a need in the art for a polyurethane composition which can form highly compatible blends with PVC and/or other thermoplast plastisols in proportions higher than 30 parts thermoplast per 100 parts polyurethane without substantial degradation of physical properties. Also there remains a need in the art for a polyurethane composition which can employ an amine curing agent while retaining adequate storage stability, i.e., shelf life.
These needs are met by the present invention.